Field of the Invention
The invention relates to a high-temperature fuel cell system having a container and having at least one high-temperature fuel cell stack disposed in the container.
A fuel cell stack includes a plurality of planar solid-electrolyte, high-temperature fuel cells, that are fixed on one another and electrically connected in series. In this case one respective bipolar plate is built in between directly neighboring cells. The plate electrically conductively connects the cathode of one cell to the anode of the neighboring cell, it guarantees gas distribution and it represents a supporting structural element.
A process which essentially represents a reversal of the electrolysis takes place in the fuel cell. The reaction partners of the combustion reaction, namely the fuel which is generally hydrogen, and the oxygen carrier which is generally air, are supplied separately. In a high-temperature fuel cell, the supply lines carrying fuel and oxygen are separated from one another in gas-tight fashion by a ceramic solid electrolyte which is provided with electrodes on both sides. During operation, electrons are given out at the electrode on the fuel side of the solid electrolyte, namely the anode, and electrons are received at the electrode on the oxygen side of the solid electrolyte, namely the cathode. A potential difference, the open-circuit voltage, is set up at the two electrodes of the solid electrolyte. The solid electrolyte serves the function of separating the reactants, of transporting the charges in the form of ions and of simultaneously preventing an electronic short-circuit between the two electrodes of the solid electrolyte. For this purpose, it must have a low electronic conductivity together with a high ionic conductivity.
As a result of the relatively high operating temperature (it is in the range from 800.degree. to 1100.degree. C.), such high-temperature fuel cells are suitable for converting hydrocarbons such as natural gas or propane that is storable in liquid form, for example, in addition to hydrogen gas, in contrast to low-temperature fuel cells. High power densities can be reached on the order of magnitude of the range of many hundreds of mW per cm.sup.2 of cell surface area, with high-temperature fuel cells. The individual high-temperature fuel cell produces an open-circuit voltage of somewhat more than one volt. Further details regarding high-temperature fuel cells can be found in the "Fuel Cell Handbook" by Appleby and Foulkes, New York, 1989.
Information regarding the way in which high-temperature fuel cells can be used, for example in combined heat and power plants, can also be found in an article entitled "Technische und wirtschaftliche Aspekte des Brennstoffzellen-Einsatzes in Kraft-Warme-Kopplungs-Anlagen" Technical and Economic Aspects of Fuel Cell use in Combined Heat and Power Plants! by Drenckhahn, Lezuo and Reiter in VGB Kraftwerkstechnik, Volume 71, 1991, Issue 4.
In a high-temperature fuel cell system, one or more stacks of high-temperature fuel cells are usually built into a container. The fuel and the oxygen carrier, usually air, are supplied in heated and slightly compressed form through external supply lines to the respective anodes and cathodes of the high-temperature fuel cells. In this case the fuel supply is usually constructed in such a way that approximately 80% of the fuel is consumed in the high-temperature fuel cells and the remaining 20% of the fuel is discharged together with the product water formed from hydrogen and oxygen ions in the reaction through pipelines. On the fuel side, the gas mixture discharged from the high-temperature fuel cells is not recirculated but instead is catalytically post-combusted, with the liberated energy being used to preheat the reactants and/or to produce steam.
On the cathode side the air volume flow is greater by approximately a factor of 8 as compared to the fuel volume flow. In order to not lose, or to only partially lose the heat content of the exit-air mixture leaving the high-temperature fuel cells in the container, it is customary to discharge the exit-air mixture on the cathode side from the container at least partially through pipelines, to recompress it and to feed it back again into the container through supply lines. In that case, however, a series of disadvantages occur: in the case of the heretofore known so-called "mono-block structure" (see Fuji Electric Review, Vol. 38, No. 2, page 58, and MBB in "Handelsblatt" of Jun. 12, 1990), very large pressure drops are produced on the distributor side and the manifold side, which is to say in the fuel-cell inlets or outlets on the air side, and those pressure drops can only be compensated for with a compressor having a relatively high power demand. Those pressure drops are usually above approximately 50 mbar.
In particular in the case of high total electrical powers of the high-temperature fuel cell system it is easy to recognize that considerable problems exist on the cathode side due to the multiplicity of supply lines and discharge lines and due to the gas compressor. The gas compressor must compress a hot, oxygen-containing exit gas on the cathode side, which causes particularly high maintenance expenditure, especially for the moving parts of the compressor. In order to avoid that disadvantage, German Published, Non-Prosecuted Application DE 40 21 097 A1 discloses first cooling the exit gas on the cathode side to below approximately 650.degree. C., and then compressing and subsequently reheating it. Disadvantageously, that configuration requires the use of additional heat exchangers and the introduction of additional quantities of heat. In addition the flexurally non-rigid routing and the fitting together of the multiplicity of individual pipes on the supply and discharge sides of the cathodes are difficult.